Anomalous propagation effects can cause a large number of unwanted returns in radar systems subject to certain environmental conditions.
For the purposes of this application, the term anomalous propagation covers the different electromagnetic wave propagations not encountered in a standard atmosphere that refer to cases when a signal propagates below the normal radio horizon.
For example, when a radar system 1 is located in a hot and humid environment, this environmental condition causes radar pulses to be transmitted over very long distances and below the horizon. These pulses then reflect from objects located far from the radar system, over the horizon, such as oil rigs or mountain ranges which were never anticipated to be part of the radar return.
In typical atmospheric conditions, one can normally assume that an electromagnetic wave 2 moves through the troposphere in air that decreases in temperature in a predictable way as height increases as illustrated in FIG. 1. If this is not the case, then the electromagnetic wave will follow a different path, which can lead to super-refraction or sub-refraction.
In certain situations, it can be the case that a layer of air can be cooler than the air above it, breaking the above assumptions for typical atmospheric condition. This situation is sometimes termed a “temperature inversion”, and an example of this situation is where a first layer of air near the ground starts cooling at night while another layer of air remains warm away from the ground and above the first layer.
When such a “temperature inversion” occurs, the refractive index of the air increases and an electromagnetic wave passing through the affected area is subject to anomalous propagation where the wave path bends towards the Earth's surface rather than continuing up into the troposphere as illustrated in FIG. 2.
Where the “temperature inversion” is located at the surface of the Earth, the electromagnetic beam will eventually hit the surface and a portion will reflect and be received by the radar system and the remainder will continue in the forward direction, be refracted downwards again, and hit the earth's surface again at a longer range . This may continue many times. Alternatively, where the “temperature inversion” is away from the Earth's surface, for instance in a zone where a cooler and a warmer mass of air collide, the electromagnetic beam can have its path bent within the layer of air such that it extends the distance the beam travels, possibly beyond the expected transmission distance.
The extreme of this situation is when the “temperature inversion” is very strong and shallow, such that the electromagnetic beam is trapped within the “temperature inversion” layer and the beam stays within the layer as it would behave in a waveguide. This is usually termed “ducting”. This is illustrated in FIG. 3.
In surface-based “ducting”, that is to say where an electromagnetic beam is trapped in a “temperature inversion” layer near the surface of the Earth, the beam will repeatedly reflect from the ground and then from the “temperature inversion” layer. This will cause return echoes every time the beam reflects from the ground.
The net effect of any of these anomalous propagation conditions on the performance of the Radar is that signals received at the antenna, which could normally be assumed to be returns or reflections from an object at a certain range, could actually be reflections from an object positioned significantly further away and possibly even below the usual Radar horizon. Such returns are termed anomalous targets or clutter, are range ambiguous, and can interfere with the normal processing of received signal, meaning that potential targets of interest can be lost amongst the anomalous signals. This can have an adverse effect on the performance of the Radar system and can potentially place it in danger in the event that one of the missed targets is actually a threat.
In a marine setting, where the Radar is installed on a ship, examples of the kind of objects which could cause such returns include land masses or shorelines, oil rigs, aircraft or large slow-moving vessels, such as tankers.
An illustration of the problem is shown in FIGS. 4a and 4b. FIG. 4a shows the magnitude of pulse returns in ten adjacent range cells, for a burst of 8 pulses. Each cross represents a return of a given magnitude. The plurality of diamond shapes positioned approximately between 10 and 20 dB represent the range cell boundaries.
It can be seen that in each range cell, the first 4 returns have a low magnitude—approximately between 0 and 10 dB. The next 4 returns are significantly higher—approximately in the range 30 to 45 dB. In each range cell, the last 4 returns can be attributed to anomalous clutter.
FIG. 4b shows the outputs of the fast channel filter for the corresponding range cells shown in FIG. 4a. The magnitude, after filtering, for most of the range cells is quite high and if any targets were present in the respective range cells, it would be difficult or impossible to identify them amongst that magnitude of clutter. The horizontal line on FIG. 4b at about 14 dB represents the detection threshold. Any returns above this level will be identified as possible targets.
A prior art solution to the problem is to use so-called guard pulses, which are transmitted from the Radar ahead of the normal pulses which are to be processed by the Radar. As in the example discussed above, if anomalous clutter is present in the fifth and subsequent receive periods, then 4 guard pulses can be transmitted ahead of the normal pulses. Any signals received in the first 4 receive periods, corresponding to the guard pulses are effectively ignored and only the subsequent pulses are processed and treated as valid signals. The returns from the anomalous clutter are processed by the receiver using coherent filter processing, which is able to ensure that such returns are effectively discounted.
This is illustrated in more detail in FIGS. 5a and 5b. These figures show the same range cells as are shown in FIGS. 4a and 4b but, in this case, 4 guard pulses have been transmitted in advance of the 8 pulses which are represented. The returns from the 4 guard pulses are not shown, since the returns from these are not processed and are effectively ignored. Now, all the returns from the last 8 pulses that are transmitted in the burst of 12 pulses include returns from the anomalous clutter. All the returns are of approximately equal magnitude in each given range cell.
Now that all the clutter information is available, the coherent filters are much better able to filter out the anomalous clutter resulting in far lower outputs than are shown in FIG. 4b, where guard pulses are not transmitted. This is illustrated in FIG. 5b which shows the output from the fast channel filter after the pulse synthesising algorithm has been applied. The effect of this is that any real targets located in the range cells in question are far more likely to be properly identified and processed accordingly, as they will not be swamped beneath the clutter signals
Even though the use of guard pulses is effective in dealing with anomalous clutter, the transmission of extra guard pulses wastes valuable Radar time which, in the case of a Multi-function Radar (MFR), could be better used performing other tasks. In this way, the overall performance of the radar system can be adversely affected.
It is an aim of embodiments of the present invention to address shortcomings in the operation of Radar systems in anomalous propagation conditions, whether mentioned herein or not.